Here is a method to bring a Maya cam into TG using the NukeCameraExport mel script available on High End (now Creative Crash).
In Maya
-Change camera rotation to ZXY
-Scene in CM
-Source NukeCameraExport to run
In TG
-Import OBJs from Maya at default settings ( scale at 1.0 and interpret as CM unchecked )
-Use vertical FOV and copy Focal Length settings from maya ( or make a chan file ) and then paste into "Focal Length in MM" under the camera node in Terragen.
Tested on Terragen 2.1.22.0
Good luck!
-Ethan